There is conventionally known a developer cartridge for an electrophotographic-type image forming apparatus that forms a developer image on a recording medium. Japanese patent application publication No. 2013-37345 discloses one such developer cartridge provided with a developer bag (i.e. flexible developer container). The developer bag accommodates toner (i.e. developer) therein and is disposed inside a casing of the developer cartridge. An opening of the developer container is sealed with a sealing member when the developer cartridge is shipped. When the developer cartridge is used for the first time, the sealing member is peeled off by an opening member, and the opening is irreversibly open. The opening member also functions as an agitation member for agitating developer.